1. Field of the Invention
This invention relates to a granulation water control apparatus for a granulating machine which is used in a granular fertilizer production process and to its granulation water control method.
2. Description of the Prior Art
The process for producing a granular fertilizer which has nitrogen fertilizer, phosphoric fertilizer and the like as fertilizer components contains a granulation process using a cylindrical granulating machine. This granulation process granulates by rotating the granulating machine body while feeding fertilizer raw material, water and the like into the body.
In such a granulation process, it is necessary to adjust the mixing ratio of fertilizer raw material and water (granulation water) in order to produce a fertilizer with good yield. This adjustment of the mixing ratio has been conventionally effected depending on the experience and perception of an operator. But, Japanese Patent Application Laid-open No. 1-275490 discloses a technique which adjusts the mixing ratio automatically. This disclosed technique will be summarized as follows.
(a) A weight ratio which is seen when granulation is ideally effected is set previously. PA1 (b) The granulated fertilizer actually discharged from a granulating machine is measured for its particle size to calculate a weight ratio. PA1 (c) The weight ratios in the above (a) and (b) are compared to judge granulation conditions. PA1 (d) According to the judgment of the above (c), a quantity of the granulation water which is fed to the granulating machine is decided.
The weight ratio is a ratio of weight of a granulated fertilizer with an allowable particle size range, weight of a granulated fertilizer with a particle size smaller than the allowable particle size range, and weight of a granulated fertilizer with a particle size larger than the allowable particle size range.
Specifically, when the granulated fertilizer tends to have a larger particle size, the quantity of the granulation water is decreased. When the granulated fertilizer tends to have a smaller particle size, the quantity of the granulation water is increased.
In case of judging the granulation condition based on the weight ratio, however, there is a drawback that a yield is not good because a change in granulation condition cannot be detected quickly. For example, as shown in FIG. 7, suppose a case that an allowable particle size range is 1.75 to 4.5 mm. Even when a state that the central particle size is almost at the intermediate point between 1.75 mm and 4.5 mm as shown in FIG. 7A has a tendency to change to a state that the central particle size becomes a larger size as shown in FIG. 7B, the weight ratio itself does not change very much. Then, when the change in weight ratio is detected, recovery of a yield takes substantial time in view of the characteristics of granulation control (requiring a certain time from charging of raw material to taking out a product) because the timing of controlling has been already delayed. As a result, a large decline of the yield is caused.
And, the above technique also has a drawback that accurate control of the granulation water cannot be effected because the change in granulation state is not correctly reflected in the weight ratio. For example, when the granulated fertilizer changes from a tendency falling in a large size to a tendency becoming a very small size, the weight of the granulated fertilizer with a large size and that of the granulated fertilizer with a small size increase simultaneously, so that the change in granulation condition does not appear in the weight ratio.
Further, there is a case that the granulation condition is quite different in weight ratio from that obtained when the granulation is ideally made due to differences in pulverizing condition of solid raw material and conditions such as temperature, humidity and the like. When the control aiming at the above weight ratio is effected under such poor conditions, there is a drawback that the control is biased because the pulverization condition cannot be put close to the target condition even after the lapse of time. When such a biased control is effected, in the worst case, the fertilizer becomes like mud or sand, resulting in a uncontrollable state.